The successful treatment of deep-seated malignant tumors within a patient is often a difficult task. The objective of the treatment is to reduce in size or completely remove the tumor mass by one or more modalities available at the treatment facility. Common treatment modalities are surgery, chemotherapy, and x-ray therapy. One treatment modality used alone or in conjunction with one of the above modalities is "tissue heating", or hyperthermia. Hyperthermia can be considered as a form of high fever localized within the body. A controlled thermal dose distribution is required for hyperthermia to have a therapeutic value. Typical localized-hyperthermia temperatures required for therapeutic treatment of cancer are in the 43.degree.-45.degree. C. range. Normal tissue should be kept at temperatures below 43.degree. C. during the treatment. Typically, hyperthermia is induced in the body by radio-frequency (RF) waves, acoustic (ultrasound) waves, or a combination of both. One of the most difficult aspects of implementing hyperthermia, with either RF or ultrasound waves, is producing sufficient heating at depth. Multiple-applicator RF hyperthermia arrays are commonly used to provide a focused near-field main beam at the tumor position. Ideally, a focal region should be concentrated at the tumor site with minimal energy delivered to surrounding normal tissue.
In RF hyperthermia systems, the hyperthermia antenna beamwidth is proportional to the RF wavelength in the body. A small focal region suggests that the RF wavelength be as small as possible. However, due to propagation losses in tissue, the RF depth of penetration decreases with increasing transmit frequency. One of the major side-effects in heating a deep-seated tumor with a hyperthermia antenna is the formation of undesired "hot spots" in surrounding tissue. This additional undesired heating often produces pain, burns, and blistering in the patient, which requires terminating the treatment immediately. The patient does not receive anesthetics during the hyperthermia treatment in order to provide direct verbal feedback of any pain. Thus, techniques for reducing hot spots while maximizing energy delivered to the tumor site are desired in hyperthermia treatment.
RF hyperthermia systems with electric field transmitting arrays, i.e., antenna arrays, in the frequency band of 60-2000 MHz have been used to localize heating of malignant tumors within a target body. Phase control alone of the transmitting antennas of such an array has been used to synthesize therapeutic RF radiation patterns within a target body. Theoretical studies of adaptive control of individual antenna phase and power (transmit weights) has been used to maximize the tumor temperature (or RF power delivered to the tumor) while minimizing the surrounding tissue temperature (or RF power delivered to the surrounding tissue). Invasive temperature measuring techniques have been used to optimize the radiation pattern within a target body.
One commercially available hyperthermia annular phased-array antenna system is the Model BSD-2000, SIGMA-60 applicator, available from BSD Medical Corporation, Salt Lake City, Utah. This phased-array system fully surrounds the patient, placing the patient at the center of an annular array of dipole transmit antennas. By fully surrounding the patient with an annular phased-array, it is possible to obtain constructive interference (or signal enhancement) deep within the target volume. This hyperthermia system uses a 60 cm array diameter with eight uniformly spaced dipole elements operating over the frequency band 60-120 MHz. The eight dipoles are fed as four active pairs of elements. There are four high-power amplifiers which drive the dipole pairs with up to 500 W average power per channel. Each of the four active channels has an electronically controlled variable-phase shifter for focusing the array. Temperature and electric-field probe sensors (both invasive and non-invasive) are used to monitor the treatment. A cool-water (5.degree.-40.degree. C.) bolus between the patient and the phased-array is used to prevent excess heating of the skin surface. The water bolus is filled with circulating distilled water, which has a very low propagation loss.